With the types of data and methods of data sending having become diversified, recent years have seen a need for a particular data sending system, in which plurality of data sending units share paths for high-speed data transmission through which each unit transmit the data interactively, arise. To meet the need, paths for high-speed data transmission, such as bus-shaped or ring-shaped paths, wherein plural data sending units and the data receiving units receiving the sending data are connected to the path, are employed.
FIG. 1 shows an example of an image data sending system. As shown in FIG. 1, a network control unit 1 performs the control and management over the whole ring-shaped network (dual optical ring network). In this example, a control unit 2 controlling and managing the network, and a data receiving unit 3 receiving the image data which is transmitted through a ring-shaped network 8, are provided in the network control unit 1. The former control unit 2 performs the control and management over the whole network according to an instruction of operators given through a console unit 6, or a network control and management program stored in the control unit 2.
The latter data receiving unit 3 is permanently allocated to a predetermined logical path (channel), where it receives the data transmitted from one or a plurality of data sending units 4-1˜4-3 and displays the received data on a monitor 7. Each data sending unit 4-1˜4-3 is allocated a logical path, for communication, through which the data sending unit transmits data to said data receiving unit.
Said each data sending unit 4-1˜4-3 shares a plurality of logical paths, to each of which each unit can send data in a free manner. The plural data sending units 4-1˜4-3 send each image data, which is collected by each camera 5-1˜5-3 provided to each unit 4-1˜4-3, to the data receiving unit 3 by inserting the image data into the logical path of said data receiving unit 3 in a time sharing order.
FIG. 2(a) and FIG. 2(b) show an example of the image data which is transmitted in the ring-shaped network described in the FIG. 1.
FIG. 2(a) represents an example of a data sending format. Video signals from each camera 5-1˜5-3 are converted into a variety of standardized video data signals (in this example, data signals with transmitting speed of 1,536 Mbps or 4,608 Mbps) and are stored sequentially in the payload section (payload) in a SDH signal flowing through the dual optical ring network 8. Each SDH signal mentioned above is to be received by the data receiving unit 3 as a predetermined logical path signal.
FIG. 2(b) shows an example of image data which is inserted alternately into the logical path of the data receiving unit 3. This example represents transport streams processed by MPEG 2 image system (MPEG 2-TS), wherein no data to be transmitted exists in a time slice between t2˜t3 for data switching. Conventionally, unspecified data is transmitted when no data to be transmitted existed (all data sending units output whatever they recognize an input). When the data sending unit, to which the time slice is switched, has no data to send, a time span occurs at which no data is inserted into the time slice. Consequently, an unspecified data (NULL, as a kind of unspecified data) exists in the logical path during that time span. Thus, conventional system has a problem in such a case that a parity error occurs in the data receiving unit receiving sending data, which determines that there is a circuit abnormality in the path for high-speed data transmission.